When the spacing between building frames exceeds a distance where a cold-formed rolled section is no longer sufficient to carry the applied loads, the building supplier must use a different roof structural member to carry the environmental and surface loads applied to the structural members of the roof. In most circumstances, the roof structural element selected is a bar joist member which is known as a “Warren Truss.”
In a conventional steel joist system, such as used in large-scale buildings, illustrated in schematic form in FIGS. 1 and 2, open web steel joists 10 rest on structural supports such as beams or on load-bearing walls 12. Wall 12 may be constructed of steel studs, red-iron, brick, block, poured concrete or other such material. Joists 10 have a bottom chord 14 and a top chord 16, connected by a plurality of web members 18. Bottom and top chords 14 and 16 generally comprise angle irons welded to web members 18. Top chord 16 typically has a further pair of angle irons welded to its underside at both ends, together forming joist shoes 20 which rest upon top surface 13 of wall 12. When in place on wall 12, joists 10 are generally parallel. Although joists 10 extending in opposite directions from wall 12 may be longitudinally aligned, they are preferably staggered, as shown in FIG. 2. Typically, adjacent joists are spaced apart from center to center. Joist shoes 20 space the top chord 16 above top surface 13 of wall 12. Typically, a corrugated metal pan or decking 22 (shown in FIG. 1) rests on top of top chords 16 of joists 10, and may be secured thereto by any suitable means such as welds or screws.
When bar joists are used, they create several problem areas that the metal building supplier must accept or be able to consider in his or her building design. The metal building companies have no control of the economics of the bar joist design, simply because they are not designing or manufacturing the bar joist. The bar joist industry is a mature industry with little motivation to work more closely with the metal building companies to develop a better product because the purchases of bar joists by metal building companies constitute a very small segment of the total bar joist industry. The basic bar joist design does not work very well with some metal building products, particularly with the standing seam roofs that are available in the construction industry.
The typical bar joist assembly, such as shown in FIG. 3 in an exploded fashion, includes hot rolled angles used for the joist chord members, and hot rolled angles, rods and Cee sections for the web members. Even though there is a large selection of hot rolled angle sizes available in the marketplace at any one time, a bar joist fabricator may only carry a limited number of different angle sizes in inventory. Due to this limited flexibility in inventory, a change in cost and weight can be significant when increasing the joist size to provide the additional load carrying capacity because the designer has to go to a deeper bar joist depth or use the next available angle size in the inventory. This situation makes the efficient design of the bar joist difficult to control for a specific metal building design.
FIG. 3 shows, in particular, the components of the bar joist 10 of the prior art. Joist assembly 10 includes a pair of angles 24 and 26 which are welded together to form the top chord 16. Similarly, separate angles 28 and 30 are welded together to form the bottom chord 14. The web members 18 comprise a plurality of separate members 32 that are placed in angled relationship between the top chord 16 and the bottom chord 14. Joist shoes 20 are affixed to the opposite ends of the top chord 16. FIG. 4 shows, in particular, the manner in which the separate angles are welded to the bottom chord 14. In particular, weld 34 must be applied between the respective angles of the bottom chord 14 so as to secure members 32 in their desired orientation. FIG. 5 shows how the angles 28 and 30 are welded together with members 32 in the assembly of the bar joist assembly 10.
Some bar joist manufacturers create a framework to hold the individual bar joist pieces (such as those shown in FIG. 3) in their proper position for the final joist assembly. Some manufacturers do not use frameworks for assembly and depend upon operator accuracy in establishing the joist dimensions. The joist measurements are used only in the setting up of the framework. Once the framework has been assembled, the framework is not remeasured during that bar joist's production until the next joist shape or depth is to be produced. If during the use of the framework, the framework gets out of adjustment, the measurements of the individual bar joist piece locations are not rechecked during the assembly process unless there is an obvious problem.
Since the joist is made up of a series of individual pieces, if any individual pieces are not correctly formed, as long as they fit within the framework, the variation in individual section length may not be noticed. The end result is that an incorrectly dimensioned part used is in the overall joist assembly. The individual bar joist pieces are preassembled in the framework. If care is not taking during the positioning process or if the framework gets out of alignment, the individual pieces may not be properly positioned for the final assembly. After the individual pieces have been clamped together with separate clamps at each joist panel point, the unit is moved to another location for finish welding. The clamps can be knocked loose during this handling process. As a result, the individual parts can move and create incorrect dimensions in the final joist assembly.
Because of the multitude of individual pieces, the welds between the chord and the web members are the only way that the joist loads can be transferred through the joist. The failure of one weld in any location may create a complete joist failure. The individual chord members are welded together with manual welds at each joist panel point. Because the welds are not all done at the same time, some welds will start to cool while other welds are still being applied. Because of the time delay in the application of these welds, the differential cooling process can create distortional bends in the chord length at the joist panel points. This will make it difficult to keep the chord straight. The amount of manual handling and welding will generate considerable labor costs as well as in making the assembly difficult to control from the quality control viewpoint. It is very difficult to hold the required dimensions.
With respect to field installation, the bar joist chords include two hot rolled angles which are attached at the joist panel joints with welds. A number of problems will occur because the chord includes two angles that are not continuously attached along their entire length. Since the angles are only connected at the panel points, the angle between the panel points is free to deflect and move sideways between the panel points. This condition creates a low lateral strength in the vertical direction of the joist. With low lateral strength, if the erector is not careful in how the joist is lifted during the erection process, the joist is prone to bend sideways easily. This will develop major kinks or bends in the chord sections. These kinks and bends cannot be easily removed.
The bar joist chord's low lateral strength will also require the use of more horizontal bridging brace members on the bottom chord in order to maintain stability under compression due to uplift loads. If the chord is not adequately restrained, the load carrying capacity of the joist decreases significantly.
When the bar joist members were initially developed, the roof covering was attached to the structure by welding it to the bar joists. The double angles in the top chord were not a problem using this installation method since the entire top surface of the top chord angles is available for attachment welds. However, builders have begun using standing seam roof covering systems which require the use of a connector clip. When the connector clips began to be used, a problem developed because the connector clip is made of a thin material which has to be screwed to the bar joist top chord. The available top chord surface for the screw attachment of the roof system clip is a much smaller component of the total chord surface because a screw cannot be installed in the gap between the bar joist angles nor in the fillet area of the individual hot rolled angles in the chord.
The angles used in the chords are made by a hot rolling process. This production method causes the actual thickness of the angle legs to be usually greater than the specified design thickness in some portions of the angle length because the producer will use the design thickness as the minimal acceptable thickness in order to ensure that enough material is provided. The hot rolling process can create the development of “hard spots” which are localized spots with high material stresses that develop during the cooling of the product after it has been formed. These resulting hard spots are difficult to drill into and may require the use of a heavier screw type or the hand drilling of the hole to install the screw. Both of these solutions increase the installation cost of the roof system on the bar joists.
Since the joist web member is either a rod, an angle or a Cee section, and since the chord section is made up of angles with their legs turned inward on the joist, there is no flat surface on the inside of the bar joist to attach the bridging brace with a screw. As a result, it is necessary to carry out welding. The welded attachment of the bridging brace angle can only be carried out by field welding. This will require a qualified welder. Such qualified welders will often work at higher salaries than typical steelworkers and can only work when the weather conditions will allow electric welding. The frame flange braces which are used to stabilize the main frame cannot be easily attached to the bar joist webs and chords unless a weld attachment is used. As such, existing joists require extensive use of welding activities.
It is an object of the present invention to provide a cold-formed joist assembly which minimizes the amount of welding required for the formation of the joist.
It is another object of the present invention to provide a joist assembly which facilitates the use of automated welding processes.
It is a further object of the present invention to provide a joist assembly which is stronger in the horizontal direction which reduces the amount of top and bottom chord bridging brace locations for lateral bracing requirements.
It is a further object of the present invention to provide a joist assembly which reduces the likelihood of bending during the handling in an erection process.
It is a further object of the present invention to provide a joist assembly which provides straighter chord lengths for the installation of standing seam roof cover attachment clips.
It is a further object of the present invention to provide a joist assembly which minimizes the possibility of injury during the assembly and installation of the joist assembly.
It is another object of the present invention to provide a joist assembly which facilitates the application of screws into the chord section.
It is another object of the present invention to provide a joist assembly which eliminates the requirements of field welding and the cost associated with field welders during the installation of the joist assembly.
It is still another object of the present invention to provide a joist assembly which is easy to use, easy to manufacture and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.